The oscillometric method is based on the principle that the blood vessel will be the most flexible (This state is called the “unloading state”) and will be flattened separately when the external pressure of the artery is equal to the mean blood pressure and higher than the internal systolic blood pressure. Since the internal blood pressure of the artery is changing periodically along with the heart beat at all time (during a heartbeat cycle, the highest pressure is called systolic blood pressure, the lowest pressure is called diastolic blood pressure, and the average of the all pressure value over the heartbeat cycle is called the mean blood pressure), and the diameter (or volume) is changing periodically with the blood pressure so as to form the arterial pulse. The amplitude of the pulse will be maximum when the external pressure of the vessel is equal to the mean blood pressure so that the vascular wall is the most flexible, and the amplitude of the pulse will disappear when the external pressure of the vessel is higher than the systolic blood pressure so that the blood vessel is flattened. When measuring the blood pressure by using oscillometric method, at first, an air bladder (or liquid bladder) to apply the external pressure to the artery is fixed on the skin over the artery. Then the pressure of the bladder is changed. At the same time, the change in pulse amplitude is measured by a pulse transducer during the entire course. If the bladder pressure can be transmitted accurately to the outside of the blood vessel through the soft tissues under the center of the bladder and the pulse transducer can detect the arterial pulse from these soft tissues, the pressure will be equal to the mean blood pressure and the systolic blood pressure separately when the pulse amplitudes are at their maximum and close to disappearance. Therefore, the mean blood pressure and the systolic blood pressure can be measured by measuring the bladder pressure at the two moments using a pressure transducer. Furthermore, the diastolic blood pressure can be obtained by using some estimation algorithm. This method can only measure blood pressure intermittently because each process of pressure change for one cycle of measurement of the blood pressure needs a considerable amount of time.
Vascular unloading method is based on the principle that the diameter of the blood vessel will not change with the wave of the blood pressure in the vessel (or will not pulsate), but will maintain at its unloading state when the pressure outside the vessel is equal to the internal blood pressure at any given time. This method includes an air bladder (or liquid bladder) that applies the external pressure to the artery and a pulse transducer, and a feedback control system which uses the measured artery pulse to control the pressure of the air bladder. When measuring the blood pressure continuously by using vascular unloading method, at first, like the oscillometric method, the air bladder pressure is changed in a certain range, and at the same time, the change in pulse amplitude in the bladder pressure is measured. When the bladder pressure is equal to the mean pressure in the artery so that the vascular wall is the most flexible, and the amplitude of the pulse is the highest, the feedback control system is connected to magnify the measured signal of the pulse and phase compensate. The feedback control system is further used to control the bladder pressure so that it will change according to the pulse wave on the base of the mean pressure. Once the wave of the pressure on the outside of the blood vessel is made the same as the wave of the periodical change of the blood pressure inside the artery, both in shape and amplitude, so that the force on both the inside and outside the vessel wall reaches a dynamic balance, the diameter of the artery vessel, instead of changing with the wave of the intra-vascular blood pressure, will be maintained at its unloading state; i.e. the pulse oscillation amplitude is near zero. At this time, if the bladder pressure is continuously measured by a pressure transducer, the continuous measurement of the instantaneous blood pressure (i.e. blood pressure wave) can be obtained.
The two methods mentioned above are not currently used on the upper arm where the blood pressure is normally measured, but on the finger to measure the blood pressure of the finger artery. This is mainly because that the position of the brachial artery of the upper arm is very deep, so that the external pressure must be applied to the upper arm from all-rounded or near all-rounded of the arm, so as to transmit adequately the external pressure to the brachial artery. Because of the pressure, long-term frequent use of the oscillometric method to measure the blood pressure intermittently or keeping using the oscillometric method to measure the blood pressure continuously will all seriously affect the blood circulation and nerval function of the entire lower arm and the hand. However, the position of the finger artery is shallow, when measuring the finger blood pressure, the influence on the blood circulation and nerval function of the finger, caused by the increased bladder pressure, is smaller.
Numerous clinical experience results have shown that the two methods have another big problem when the finger is used for measuring blood pressure, that is, because the finger artery belongs to distal arteriolar, comparing with the so-called “system blood pressure” (or the blood pressure of the aorta near the heart) that is used clinically when judging whether the patient's blood pressure is normal or not, blood pressure of finger is around 10 mmHg lower under normal conditions. In case of arteriosclerosis, the difference can reach several ten mmHg. More importantly, because the composition of the smooth muscle inside the small artery vessel wall is larger than that in the aortal wall, and these vascular smooth muscle are very easily affected by various factors (such as coldness, anesthesia, etc.) so as to either produce vasoconstriction or vasodilation, which causes the blood pressure in the small artery to wave at a great range, under many circumstances, the blood pressure obtained from finger artery cannot be used to reflect the system blood pressure of the patient. Especially when the patient's circulation function is very weak, the finger artery can sometimes cause the loss of blood in the artery due to the extreme vasoconstriction of the vascular smooth muscle, so the blood pressure cannot be measured on the finger.
In order to correctly reflect the system blood pressure while not affecting the blood circulation of the distal part of the measured area, a proposal to change the measuring position of the two methods to the wrist has recently been made, and also changing the traditional all-rounded pressure bladder to a local pressure bladder so as to only give pressure to one of the two arteries in the wrist (radial artery and ulnar artery) has been considered. This is based on two points: first, the diameter of the radial artery or ulnar artery is much bigger than the finger artery, and the composition of the smooth muscle in the vessel wall is less than in the finger artery, so their blood pressure is closer to the system blood pressure than that of the finger artery, and also not easily affected by other factors. In addition, even when the patient's circulation function is very weak, the pulse can be always detected from the radial artery or ulnar artery, making the measurement of the blood pressure possible.
Secondly, normal people have more than two bigger arteries and veins on the wrist, of which, two arteries (radial artery and ulnar artery) are connected to each other by two arterial arches in the palm; several veins on the back of the hand are also connected to each other by the vain web on the back of the hand. The connection of the these blood vessels guarantees that, even if one artery and/or part of the vein is occluded (has an occlusion) for a long time, but the other artery and the rest parts of the veins will still have a smooth blood flow, the circulation of the hand will basically not be affected. Therefore, frequent and continuous blood pressure measurement can be performed for a long time with these two methods on one wrist artery of either the radial artery or ulnar artery.
Although related researches have shown that the mean blood pressure, systolic blood pressure, or the blood pressure wave can be separately and accurately measured, with oscillometric method and vascular unloading method, on the radial artery near the most protuberant spot on the volar aspect of the distal end of the radius, research has also discovered that it is actually very difficult to measure the blood pressure accurately on the wrist. It is mainly because the precision of the blood pressure measurement is very sensitive to the measuring position, so even on the most protuberant spot on the volar aspect of the distal end of the radius, on various positions with a difference of only 2,3 mm among them, the measured blood pressure can be greatly different. In addition, the precision of the measurement can also be affected by outside factors. First, the measured blood pressure will vary greatly when the wrist turns with the long axis of the forearm as the axis of rotation, or when the hand bends towards the palm side or the back side of the hand. Secondly, along with the increased bladder pressure, the bladder might move towards not only the center of volar aspect of wrist along the circumferential direction, but also the hand along the long axis of wrist. All these movements may change the bladder volume, and the movement in the direction of circumference and long axis may also cause the pulse transducer to move its position. Among these, the position change of the pulse transducer may affect the measuring precision of the oscillometric method and the vascular unloading method.